In an electrical installation it is usual to interpose a low-pass power line filter at the input of an electrical appliance to help prevent high frequency signals from passing between a load appliance and supply, thus improving the electromagnetic compatibility (EMC) of the system. A typical low-pass passive power line filter may comprise series inductive and shunt capacitive interconnected components.
Pulse width modulation (PWM) power electronic loads, such as variable speed motor drives, generate parasitic common mode currents. This is due to the high voltage slew rate of their switches imposed on the stray earth capacitance in the system. This common mode current constitutes an Electromagnetic Interference (EMI) which manifests itself as common mode (CM) conducted emissions and earth leakage current, phenomena in the field of Electromagnetic Compatibility (EMC).
The low pass power line filter provides a degree of common mode (CM) attenuation which effectively reduces the amplitude and bandwidth of common mode current allowed to flow on the supply line. However the size and cost of the filter can be prohibitively high especially when the common mode currents are high or when significant attenuation of such current is required at or near to the drive switching frequency.